Ecological Studies in South Benfleet Creek with Special Reference to the Amphipod Genus Corophium B.A.*

Reprinted from, THE ESSEX NATURALIST for 1961

This paper is an account of the work done on some ecological aspects of South Benfleet Creek. A description of the Creek is given in the Introduction and a brief account of the methods used in the second section. The ecology of the most important species found in the Creek is described. The Creek is most interesting with regard to the distribution of the two species of Corophium found there, one of which, Corophium arenarium, is a relatively rare species. It has been found that the composition of the substratum, and associated factors such as water content, is the dominant factor in determining the distribution of the two species of Corophium and of the other two most important species, Nereis diversicolor and Scrobicularia plana. The important distinctions between the habitat of Corophium volutator and Corophium arenarium have also been worked out.

INTRODUCTION Benfleet Creek South Benfleet Creek (G.R. TQ 79 85) is a small tidal inlet on the north bank of the River Thames about 7 miles from its mouth at Shoeburyness. The Creek is just over 3 miles long, from Canvey Point to South Benfleet Bridge, nearly half a mile wide at its mouth and about 80 yards wide near the bridge. It runs in a S.S.E.-N.N.W. direction between the low-lying land of Canvey Island and Hadleigh Marsh. Because the surrounding land is all below H.W.M.M.T.* level, strong concrete embank- ments have been built along the whole length of the creek, on both banks. It is therefore closely confined to its channel, with no small tributaries of fresh water entering it. There is a flow of salt water in the main channel, even at low water. As can be seen from figure 2, the creek is bordered on either side by extensive saltings. These are finely dissected by narrow, deep, muddy channels, filled at high water. The main channel is deeply incised at low water in its lower reaches where it is too deep and wide to ford. Just above the island, however, the

*The material for this paper was prepared while the author was a student at the University College of North Staffordshire. He is now a research student in Marine Zoology at the University of Wales, University College, Swansea. *High Water Mark of Medium Tides. 292 THE ESSEX NATURALIST stream bed opens out and the stream is fordable. The main stream flows round the south side of this island but a small tributary stream is present on the north side, with its source at the western end of the island.

HEIGHT IN FEET 26 10

25 DISTANCE IN YARDS Figure la. The profile of the transect across area B. The stations marked on the profile are those given in Tables II and III.

PIE1G447 mreeT

•

6 4 . COROPHIum 01.V , 6•0111 20161- -

2•-

as r•• 103 04 68 42 DISTANCE 114 VAMPS Figure lb. The profile of the transect across area G.

The form of the stream as described above, is, in part, responsible for the nature of the substratum of the creek at low tide. Although it may be described generally as estuarine mud, the substratum varies greatly in detail, as will be shown later. Away from the main channel, towards High Water Mark, the muds are very soft and sticky. Nearer the main channel, and at intervals along its whole length, one finds well-defined areas of relatively clean sand which are quite firm to walk on. These 'sand banks' occur usually where there is a small meander in the stream. Just after the turn of the tide the incoming currents are swift (3 ft./ sec.) in the incised channel, too swift to allow for the deposition of the silts but sufficiently slow to allow for the deposition of the sand. As the tide rises the main stream overflows its incised channel, the current is checked and slackens, thus allowing for the deposition of the finer material. This is well SOUTH BENFLEET

......

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LEIGH ISLAND ,•■•••

ammi CHANNEL AT L.W.M.T.

=1 STONE EMBANKMENT

• jttill.fe 1.6.811 EARTH EMBANKMENT

0 bot, • 0 o• COROPHIUM VOLUTATOR cf• 0 0

COROPHIUM ARENARIUM CANVEY ISLAND

SHELL DEPOSITS

220 440 880 YARDS. SALTING

Figure 2. A map of South Benfleet Creek showing the distribution of C. voNtator and 0. arenarium and the positions of the sampling stations. Enlarged and modified from the Ordnance Survey two and half inch sheet. ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 293 illustrated in Area G (see fig. lb.) where the steeper slope up from the stream appears to be of a sandy nature and much firmer as compared with the flat top of the region, which is of a decidedly muddier consistency, Moore and Spooner (1940), Percival (1929), Rees (1940). The preliminary investigations of the lower reaches of the creek in June revealed the presence of a large population of the amphipod crustacean Corophium. An investigation of the whole creek, however, revealed that there were two species of Corophium present, each species having a different distribution. The aim of this paper, therefore, is to discover reasons for the differing distribution of these two species—Corophium volutator (Pallas) and Corophium arenarium (Crawford)--in Benfleet Creek.

Position of the sampling stations Sampling stations were set up in each type of substratum in the Creek. On the sand banks A, B and E—C. arenarium areas—random samples were taken. Their position is shown on figure 2 by small circles. In addition, a transect was put down on bank B and sampling stations were set up at 1 yard intervals from Low Water Mark on August 7th, 1959 (see figure la). The position of this transect is shown in figure 2 by a line across area B. Nine other transects 20 feet apart were also surveyed and the positions of the C. arenarium zone marked. Some of these transects are used in figure 6. Another transect was put across area G, the C. vo/utator zone, about mile below Benfleet Bridge. The transect was surveyed (figure lb) and four sampling stations established 8, 50, 115 and 125 yards from H.W.M.M.T. The position of the transect is shown by the line across area G in figure 2. Three sampling stations (shown by crosses in figure 2) which have been called the Long Transect were set up outside the estimated Corophium zones. It was important that this should be done in order to complete the ecological picture and to discover factors limiting the distribution of Corophium. Animal communities are subject to rapid changes, caused by the yearly life cycle of the species, Watkins (1941), and possible changes in the location of the community at different seasons of the year. Thus the time at which any investigation of an animal community is made is of some importance. Two preliminary investigations of the area were carried out, one at the end of June and the other at the end of July. The detailed work was done during the period of August 1st to August 29th, 1959. Final details, mainly concerned with collecting water and some soil samples were completed up to September 7th and some soil samples had to be recovered and the pH of the water determined early in December. Thus the details of distribution of the species, and the conclusions arrived at as to the factors determining their distribution, can only be related to the month of August 1959, 294 TIIE ESSEX NATURALIST and further work, completed over a number of years would be necessary if these conclusions were to be confirmed.

METHODS Specimen Sampling—In two areas, B and G, transect sampling was used. All other samples taken were random samples. Each sample covered an area of 100 sq. ems. to a depth of 15 ems. The figures in Table IV are obtained from two such samples at each station. Water Content—A core sample of the substratum at each station was obtained by using a glass tube 5/8th of an inch in diameter and 10 inches long. This was screwed into the substratum and a rubber bung inserted in the top of the tube. The tube was then screwed out again and another bung inserted in the other end, Fraser (1932), Stopford (1951), Southward (1953). The water content of one inch of the core from half inch below the surface was estimated in the normal way. Soil Analysis—Core samples were obtained in the same way as above, air dried and analysed later in the laboratory. The method described by Rees (1940) was found to be unsatisfactory and a more refined method, in which subsamples are taken from a soil suspension by pipette, was therefore used. This method will be described in detail in another paper. Salinity—The amount of sodium chloride was estimated by the usual method of titrating — silver nitrate against 10 mls. 10 of seawater to which a few drops of potassium chromate had been added. Total salt content was obtained by consulting Hydro- graphical tables—Knudsen (1901—reprint 1953).

PHYSICAL AND CHEMICAL DATA Salinity TABLE I Urns Urns Total salt Location Tide Cl I Ltr. NaCl I Ltr. contenb in, 0100 Leigh Island High 17.16 28.30 31.00 Low 15.92 26-35 28.77 Benfleet Bridge High 16.66 27-42 30.05 Low 15.22 24.45 27.50 Westcliff Front High 17.83 29.40 32-21

The above table shows that there is little variation in the salinity of the water with distance up the Creek, just over 1 ° / oo 0 at low water and just less than 1 /00 at high water. Benfleet Creek is therefore not a true estuary in that there is no real gradation in salinity between source and mouth. All the salinities, however, are below the full North Sea salinity of 33 0/ „ (Westcliff Sea Front being the nearest to this) as a result of the infiltration of fresh water from the Thames Estuary. ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 295 pH The pH at low water in Benfleet Creek is : — 7.7 on the Transect in Area G—Benfleet Bridge 7.6 on the Transect in Area B—Leigh Island

%Water Content TABLE II Area Station 1 2 3 4 9 15 19 21 26 A 24.3 — — — — — B 23.7 — — — — — E 28.2 — — — — — — — — Transect in B — — 31.6 — 24-7 28.4 24.9 24.6 26.4 Transect in G38.4 25.3 39.2 28.2 — — — — — Long Transect 40.3 46.5 35.0 — —

Time did not permit a complete range of samples to be taken, for example from transect in area B but a number of points are indicated by Table II. Water content of the substratum is dependent on three major factors: — 1. Period of exposure between tides. 2. Distribution of particle size. 3. Efficiency of packing. And on two minor factors: — 1. Climatic conditions at the time of sampling. 2. Degree of slope. If on each of the transects, i.e. in Area B and G, particle size and efficiency of packing were the same then the water content would increase towards low tide. This is suggested in Transect B. The slight variations from this gradation can be explained in terms of the differences in substrate composition. There is much greater variation in water content in the transect in area G, and here all three factors are important. It may be concluded that the water content of stations 2 and 3 is a reflection of particle size, or soil composition. The result obtained for station 1 is very high because it is only a few yards above low water mark and is therefore only exposed for a short period. The result for station 4, on the other hand, is very low because at the time of sampling the station had not been covered for a few days by the Neap Tides. As a result of this, and the climatic conditions for the period August 24-31st (approx. period of Neap Tides) the surface had almost completely dried out and was cracked and sunbaked. Random samples were taken in areas A, B and E in order to give an indication of the water content of these areas, and the same applies for the Long Transect. 296 THE ESSEX NATURALIST

Soil Analysis Table III shows the results of the mechanical analysis of the soil samples taken at the given stations. The last column shows that there has been a gain in weight in every sample. Theoretic- ally there should be a loss in weight of about 5% due to various causes. These causes, and possible reasons for the gain in weight of up to 5% will be discussed, along with a detailed description of the methods of analysis, in another paper.

TABLE III 0, 0 / % % % Gain Station Gay Silt Fine Sand Coarse Sand in Weight Long Tran. 1 21.8 16.1 66.0 0.1 5.0 2 57-3 42.8 1.2 1.3 3 16.8 10.9 77-1 0.1 4.9 Trans. G 1 14-3 6.6 81.2 1.9 3.0 2 15.6 7.1 81.1 0.6 4.4 3 25-0 13.4 61.9 0.3 0.6 4 20.8 16.3 64.3 0.1 1.5 Trans. B 3 13-4 7.8 80.3 0.3 1.9 9 12-4 4.2 83.4 0.2 0.2 15 16.6 9.6 76.7 0.2 3.1 21 13-7 8.2 82.3 0.5 4.7 23 11.2 5.2 86-5 0.5 3.4 26 14.3 11.8 77.6 0.2 3.9 Area A 12-8 3.0 88.0 0.7 4.5 Area B 12.2 2.4 88.7 0.7 4.0 Area E 16-8 5.3 78-3 0.8 1.2

DISTRIBUTION OF THE MACROFAUNA OF THE CREEK The species included in this account are only those which are retained in a 30 mesh brass sieve; no account has been taken of the microfauna of the creek.

List of Species ANNELIDA-Polychaeta-Nereis dicersicolor (Muller) Arenicola marina (L.) Sabellidae (tubes)

MOLLIISCA-Lamellibranchia-Cardium edule (L.) Mytilus edulis (L.) Scrobicularia plena (da Costa) -Gastropoda-Littorina littorea (L.) Paludestrina stagnalis (L.) ARTHROPODA-Grustacea-Cirripedia-Ba/anus balanoides (L.) Elminius modestus (Darwin) Amphipoda-Corophium arenarium (Crawford) Corophium volutator (Pallas) Decapoda-Carcinus maenas (L.) ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 297

DISTRIBUTION MUD-DWELLING FAUNA I AM VERY MUCH INDEBTED TO MR. G. I. CRAWFORD FOR CONFIRMING MY IDENTIFICATION OF THE TWO SPECIES OF COROPHIUM FOUND IN THE CREEK. IN HIS LETTER TO ME HE WRITES: —"I HAVE NO DOUBT THAT THE SPECIMENS FROM AREA G ARE C. VOLUTATOR AND THAT THE SPECIMENS FROM AREAS A, B, E AND F ARE THE SAME AS WHAT I CALLED C. ARENARIUM IN MY 1937 PAPER. I EXPRESS IT IN THIS CAUTIOUS WAY, BECAUSE IN THAT PAPER I LEFT THE VALIDITY OF THE 'SPECIFIC' DIS- TINCTIONS UNRESOLVED . . . . I BELIEVE, HOWEVER, THAT C. ARENARIUM IS NOW GENERALLY ACCEPTED AS A GOOD SPECIES, BUT ALL I CAN SAY IS THAT I AM SATISFIED THAT YOUR SPECIMENS ARE THE SAME SPECIES (SUB-SPECIES OR FORM) AS THE MATERIAL ON WHICH I NAMED AND DESCRIBED C. ARENARIUM". TABLE IV THE DISTRIBUTION OF NEREIS DIVERSICOLOR ARENICOLA MARINA, SABELLIDAE TUBES, SCROBICULARIA PLANA, PALUDESTRINA STAGNALIS, COROPHIUM ARENARIUM AND COROPHIUM VOL UTATOR. STATION SPECIES ND AM S SP PS CA CV AREA A 1 — — — 105 — AREA B — 1 146 AREA E — — — 125 TRANSECT B 1 — — — — — — 2 1 — — 1 — — 3 _ _ P 1 _ _ 4 — P 2 — — 5 2 P 3 — — 6 1 P 1 — — 7 — 1 P 1 — — 8 — — — — 6- — 9 — 29 — 10 _ P 64 _ 11 — — P — 69 — 12 — — 1 — 89 — 13 — — — 98 14 3 _ P ___ 55 _ 15 — — 136 — 16 — — — 93 — 17 — — — 123 — 18 P — 104 — 19 P — — 128 — 20 P — — 132 — 21 P 1 — 184 — 22 — P — — 156 — 23 — — P — — 163 — 24 7 P _ _ 7 _ 25 7 4 3 — 26 16 — 2 — — 27 3 1 — 10 TRANSECT G 1 — — 1 — — 2 — — — 2 3 10 8 136 4 2 — 5 225

LONG TRAN. 1 20 9 163 2 43 2 178 3 20 8 154 P DENOTES THEY ARE PRESENT. 298 THE ESSEX NATURALIST

Corophium arenarium This species was first collected in 1936 just off the east end of Leigh Island, about a mile east of the mouth of Benfleet Creek. As can be seen from Table IV and figure 2, C. arenarium occurs in considerable numbers in restricted localities in the Creek. The figure given for Area B in Table IV represents a population of about 1,700 per square metre'. This species is only to be found in areas A, B, C, E and F, and their positions and extent coincide almost exactly with the limits of the 'sand banks' described earlier. In areas A. E and F, C. arenarium occurs mainly on the broad flat tops of the sand banks and tends to disappear on the slopes down to the stream. Area B however is flat on the up- stream half but the downstream half constitutes quite a steep bank (see figure la). Here C. arenarium seems to have a definite distribution pattern with the upper and lower limits varying along the bank. From the figures for Transect B in Table IV it can be seen that this zone starts suddenly at station 7. Below station 7 the mud is softer than on the rest of the transect, and covered with a brown diatom scum. Thick black sulphide mud appears about 2 ems, below the surface and because it is near low water mark the water content is high. The C. arenarium zone extends up to station 26 (which, as far as can be estimated from the Southend tidal heights, is about M.S.L.) where it suddenly stops again. This station is at the top of the bank and the nature of the substratum changes radically on the other side where no C. arenarium is found. Although no transect was put down, the position seems to be essentially the same in area A—near low tide level where the mud is softer and with a high water content no C. arenarium are to be found. This also holds for areas E and F. In all these areas C. arenarium is found in small U-shaped burrows, extending down to about 5 ems, below the surface, which because of the nature of the substratum seem to be more permanent than the burrows of C. volutator. Areas A, B, E and F are all below M.S.L., but C. arenarium does not seem to be limited to the lower tidal levels in its distribution because area C extends right up to H.W.M.T. level and the corophids are still found in that part of the bank which is not covered by the Neap Tides (the surface was sunbaked and cracked when visited).

Corophlum volutator This species is the most common member of the genus Corophium round the coasts of Britain, but in Benfleet Creek its distribution is very limited. Table IV shows that C. volutator is found only in the upper part of the transect in area G. This species also seems to occur in very large numbers, the figure for

1Square metre sampling seems to have become the accepted custom in ecological literature and so some attempt has been made to give estimates for Benfleet Creek as a means of comparison with other areas. ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 299 station 4 representing about 2,000 per square metre. Figure lb shows the general configuration of area G and the extent of the C. volutator zone. Apart from the slope up from the stream the surface of the bank is roughly level. There is much surface water, the substratum being quite soft and the thick, black, foul smelling mud appearing only 5 ems, below the surface. There is a great change in the substratum on the slopes down to the stream, it is much firmer, sandier, drier and there is no surface water. It is in this region that C. volutator rapidly dies out, only two specimens being found at station 2.

Nerois diversicolor Except for Scrobicularia piano, the distribution of N. diversicolor is more general than that of any other species. It is found in small numbers in most of the Corophium areas as well as outside them, and from low to high tide levels. The figure for station 2 on the long transect represents about 250 specimens per square metre. Here the mud is very thick and wet, with the black mud very near the surface. The relative absence of N. diversicolor in the C. arenarium zone of areas A, B and E is of interest. Whether the presence of Corophium or some other factor is responsible for this is not quite certain but where the substratum is softer than in areas A, B or E, N. diversicolor is more common, becoming very abundant where the mud is very soft outside these regions. This species occurs down to a depth of about 15 cms. in the substratum.

Scroblcularia plane This lamellibranch mollusc is common locally in muddy estuaries, and is distinguished from Maconu2 balthica by its larger size (up to 50 mm. on the long axis of the shell), the greyish yellow colour of the shell, a large internal ligament and unfrilled siphons, one of which is very long. Table IV and other observations on the Creek suggest that it has a distribution similar to that of N. diversicolor. In transect B it is again common near low tide level, absent in the C. arenarium zone and present above it as the substratum begins to change. It is more common in the C. volutator zone of area G and even more common outside these regions, in the very soft muds. Thus it seems to have a similar distribution to that of Macoma baithica described by Fraser (1932) and Rees (1940). S. plana was found in greatest numbers below about 15 ems, during the period of investigation, but there is reason to believe that it moves higher up the substratum in winter. It is interesting to note that there is a vertical zonation of these three genera in the Creek. C. arenarium and C. volutator occur in the top 5 ems., N. diversicolor generally down to about 15 ems., and S. plana predominantly below 15 ems. depth. 300 THE ESSEX NATURALIST Paludestrina stagnalis Table IV shows that this minute gastropod has a very limited , distribution, occurring only on the long transect. Where it is present it is found in considerable numbers. It avoids almost completely the areas in which both species of Corophium are found. These observations seem to confirm the conclusions of Hart (1930) who observed at Whitby that 'the small gastropod P. stagnalis occurred in mud of a slightly different nature, where C. volutator was absent, and here they were found in countless thousands'. Arenicola marina On the muddy beaches of Shoeburyness this species occurs in vast numbers, the whole beach being covered by their casts. In Benfleet Creek, however, they are almost absent. Where they are present they seem to be confined to the bottoms of temporary pools which form in small hollows in the mud in a narrow belt in the Leigh Island side of area B—station 27 is on the edge of one such pool. Sabellid tubes The identity of these tubes is not certain as none were found to be inhabited. They are composed of fine sand particles, bound together by mucus and with no calcium in their composition— this suggests that they are some form of Sabellid tube. They are limited in their distribution in the Creek, being found in two distinct zones on the steep slopes of area B. Table IV shows that these belts are found roughly from station 3-7 and 18-24. They were also found in a small part of area A. The tubes project vertically about 1 cm. above the surface of the substratum and extend down to about 10 ems. depth at which level a layer of sand concretion exists, very like an iron pan.

OTHER SPECIES Carcinus maenas This species of crab could be included either here or in the above section because although it feeds and lives on the surface it rapidly burrows into the mud, just below the surface, when slightly disturbed. The most noticeable concentrations of C. maenas are in the deep channels of the saltings where the mud is extremely soft and where there is usually a small trickle of surface water. They are also to be found in the main channel at low tide. Mussel Bed The extent of this 'mussel bed' is shown in figure 2 by area D, in the vicinity of station 2 on the long transect. It is situated in the broad, flat bottom of the stream bed in a region of very soft mud. The whole area is, however, covered with a deposit of broken shells of a number of molluscs, mainly S. plana, and there are extensive deposits of small pieces of anthracite coal which ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 301 appear to have been washed away from an old cinder tip. There are three surface-dwelling species present in this area: --Mytilus edulis, Cardium edule, and Littorina littorea. Mytilus edulis occurs mainly at the eastern end of area D, their byssi being attached to other mussel shells, to broken shell deposit and to the pieces of anthracite coal. Cardium edule and Littorina littorea are present over the whole area in appreciable numbers. The shells of all these species are quite heavily encrusted with the barnacles Balanus balanoides and Elminius modestus as also are the remaining concrete piles of an old landing stage near-by. There is also a less extensive shelly deposit a little further up the Creek. It is of the same character as that described above, but there only 1 specimen of L. littorea was found, and no other living species. The explanation for the presence of these species on one bed and not on the other lies in the fact that the shelly deposit carrying the mussel bed is uncovered by the tide for about 7 hours, whereas the area carrying no surface forms is uncovered for not more than 3 hours.

DISCUSSION The most interesting observations to be made in an ecological study of Benfleet Creek are on the two species of Corophium found there. No records have been found of any other area in the British Isles where these two species occur in close proximity. C arenarium is also a relatively rare species, or sub-species, and the only work traced on its ecology is the preliminary work of Crawford (1937) and a reference to a paper by M. Chevais of Paris in `Travaux de la Station biologique de Roscofr. Therefore this section is devoted to a discussion of the factors determining the distribution of these two species in Benfleet Creek. The four factors studied in most detail with regard to this problem were the effects of salinity, water content, the composition of the substratum and slope. Salinity A number of writers on the ecology of C. volutator suggest that salinity is an important factor influencing the location of this species. Nicol (1953) writing on the Ecology of a Salt Marsh states that 'the distribution of C. volutator is determined firstly by salinity' and found adult specimens in pools of 5% salinity. Crawford (1937) found C. volutator in the Tamar Estuary living in almost fresh water and in other localities living in water of nearly full salinity. In a salinity tolerance experiment Hart (1930) showed that 'this species can endure great ranges of salinity' and by gradual dilution of the water he kept them alive in very low salinities all winter. He also showed that they could live up to 16 days when put straight into fresh water. No information is available on the survival of C. arenarium in varying concentrations of sea water. In an experiment to deter- mine this, 8 petri dishes were filled with 50 mls. of water of 302 THE ESSEX NATURALIST varying concentrations. These concentrations were made up by diluting sea water from Westcliff Sea Front (32.2 %) with tap water. Ten freshly collected specimens were placed in each dish along with a small amount of substratum. The water in the dishes was changed every two days, and every day where necessary. The time of survival of each specimen was determined at the different salinities and then the average time for survival calculated. The results were then plotted and are shown in figure 3.

TIME IN HOURS

220 INDIVIDUALS STILL ALIVE WHEN EXPERIMENT DISCONTINUED

180

160

140

120

100

32.2 24.15 16.1 8.05 4.03 3.02 2.01 0 SALINITY °/oo

Figure 3. The survival of G. arenarium in dilute seawater. ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 303 These results show that the optimum concentrations for C. 0 arenarium is about 16 / 00 and that the animal has a wide range 0 of tolerance, living for 78.8 hours in concentrations of 2 /00. A 0 0 sensitivity to slight changes in salinity between 24 /00 and 33 /00 is suggested by the steep gradient of the graph between these two points. I was unable to get C. arenarium to live for more than 0-6 hours when put straight into fresh water (as compared with Hart's 16 days for C. volutator). The experiment of Hart with C. volutator and the results of the above experiment on C. arenarium show that these two species have a wide range of tolerance with regard to salt con- centration. It is only in localities where there is an appreciable range in salinity—e.g. the Tamar Estuary, Spooner and Moore (1940)—that the distribution is influenced by this factor. Table 1 shows that in Benfleet Creek the salinity variations are almost negligible, less than 1 V 00. Despite this, the distribution of C. volutator and C. arenarium is very restricted. This would suggest that factors other than salinity are dominant in determining the distribution pattern of this area.

Soil Composition It was thought possible, from observations in the field on the varying firmness of the substratum, that soil composition was an important factor in determining the distribution of the two species of Corophium. A number of other writers, Beanland (1939), Crawford (1937), Hart (1930), Nicol (1953), Percival (1929) and Watkins (1941) have also noted the effects of substratum composition on the distribution of C. volutator, but none have attempted to define this in specific terms. In the words of Hart (1930), 'a more accurate description of the substratum is needed'. A number of soil samples, taken in various parts of the Creek, have therefore been analysed in the laboratory and the results presented in Table III. These are graphically represented in figure 4 and show striking results.

c% SILT AND CLAY 1 0 IS 20 25 30 35 40 45 SO SS 60 I . I A • • •

• •

Figure 4. The distribution of C. volutator and C. arenarium in relation to the silt and clay content of the substratum. A, C. arenarium; B, C. volutator; C, neither species. 304 THE ESSEX NATURALIST A comparison between percentage silt and clay content of the substratum for the stations at which C. arenarium and C. volutator occur shows a marked distinction between the two species. C. arenarium is found in those areas where the silt and clay content is between 146% and 26.1 %. This results in a firmer substratum of cleaner sand. C. volutator is found in areas where the silt and clay content is higher than the above, 37.1 % and 384% for the two samples taken. The substratum as a result is less firm and of a decidedly muddier nature. Only two stations were established in the C. volutator zone, thus not giving evidence of a range of tolerance in this species. It will be noted, however, that at stations 1 and 2 on the transect across area G the percentage silt and clay content falls well below 30 % , into the C. arenarium range, and is coincidental with the disappearance of C. volutator. It can be argued therefore that this provides evidence to support the view that C. volutator occurs in areas in which the silt and clay content is higher than that for C. arenarium. The silt and clay content of the areas outside the Corophium zone is generally higher than the values within this zone, being between 39% and 57%, resulting in an extremely soft substratum. When these figures are taken in conjunction with those from station land 2 in area G, it may be argued that these two figures quoted for C. volutator are representative of its habitat. The substratum composition at station 3 on the long transect is well below 39 % , but here another factor, namely the presence of thick black mud an inch below the surface, is responsible for the absence of C. volutator. The percentages of fine sand at all these stations are inversely proportional to the percentages of silt and clay because the percentages of coarse sand in the samples are for all practical purposes negligible. These results would suggest a fundamental distinction between the habitat of these two species, and a firm basis on which to explain their distribution. In his letter, Mr. G. I. Crawford mentioned that he thought C. volutator had different burrowing habits from C. arenarium in that it built less permanent burrows. I have examined the burrows of both species and they seem to be identical in form, but since C. volutator constructs its burrows in a softer muddier substratum perhaps they are more likely to collapse than those of C. arenarium, built in a firmer substratum.

Water content For reasons stated earlier the water content of the substratum is closely related to its composition. When, therefore, the distribution of these two species is plotted with respect to water content the resulting pattern should be similar to that shown in figure 4. ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 305

cy, WATER CONTENT 20 25 30 35 40 45 50 L-L--J--L-L-L-L-2--L-U-1-i-:-.L.-L-J-L-L-L-L-L-L-L-L-L-i-J..-L-LJ M • III 0 A ei•NI SI B • •

• • •

Figure 5. The distribution of C. volutator and C. arenarium in relation to the water content of the substratum. A, C. arenarium; B, C. volutator; C, neither species.

C. arenarium areas are even more closely defined by water content than by substratum composition. This species occurs only where the water content is between 23-7 % and 28-3 %. It should be noted that C. arenarium does not occur at station 3 on the transect in area B where the water content rises to 31.6% . It may be that this water content is too high for the species to burrow satisfactorily. On the other hand, C. arenarium was found in area C, part of which was not covered by the Neap Tides, having a sun-dried surface, and where the water content was probably temporarily below 23 %. In this type of substratum (muddy sand), however, the water content below the immediate surface tends to remain more constant than if the substratum were clean sand. It is probable, therefore, that the distribution shown in figure 5 is typical with regard to water content. C. volutator occurs where the water content is higher than in the C. arenarium areas, typically being about 39.2 %. I express it like this because the figure for station 4 in area G is 28-1%. The reason for this is that at the time of sampling, the substratum around station 4 was above M.H.W.N. It had not been covered for a number of days, with the result that the surface layers had dried out and were suncracked. Although 39% may be nearer the optimum for C. volutator the position at station- 4 suggests either that (1) C. volutator has a wide range of tolerance of water conditions—or (2) soil composition is more important than water content in determining the distribution. This position at station 4 is also interesting in the light of the statement by Hart (1930) to the effect that C. volutator is never found in any numbers where the rate of evaporation of the surface layers is such that they dry out almost completely. The surface at station 4 was baked hard by the sun but contained a population of nearly 2,000 per square metre. 306 THE ESSEX NATURALIST Angle of Slope and Disturbance It has been suggested by Spooner and Moore (1940) that C. volutator is exceptionally sensitive to disturbances of the ground by tidal currents. In Benfleet Creek C. volutator is found only on the flat-topped bank of area G. C. arenarium, however, was found in part of area B on a bank with an average gradient of 1 in 7. It was on this bank, therefore, that I investigated this pheno- menon of disturbance. A number of transects had been surveyed and the Corophium zone marked on them. The angle of slope between each station has been calculated and the results for the first 12 stations on four of the transects are shown in figure 6. The arrows denote the beginning of the C. arenarium zone.

ANGLES IN DEGREES 14 2 12

13 3 4

2 4 6 10 t2 3 5 7 9 II STATIONS Figure 6. The occurrence of C. arenarium in relation to the angle of slope on four transects in area B. Arrows indicate the commence- ment of the C. arenarium zone.

Figure 6 shows that C. arenarium does not appear until the steepest gradient0 has been passed. In transect 1 the steepest slope is 13 17' at station 6 and the Corophium appears at station 8. On transect 2 the steepest gradient is 11° 46' at station 7 and Corophium appears at station 10. A similar situation holds for the other transects. This suggests that, all other factors being ECOLOGICAL STUDIES IN SOUTH DENFLEET CREEK 307 equal, C. arenarium also is very sensitive to disturbances by tidal currents. This is borne out by observations of the area round the sampling stations which had been trampled. The corophid population had disappeared from these areas completely by the following day.

CONCLUSIONS 1. The composition of the substratum, i.e. the relative percentages of fine sand, silt and clay, appears to be the chief factor responsible for the distribution of C. arenarium and C. volutator, the latter occurring in areas of a slightly muddier nature than the former. 2. The percentage water content is also an important factor influencing distribution. From evidence put forward in the discussion it seems to be a secondary factor to substrate composition. Where an area is fairly homogeneous with regard to substrate composition, e.g. transect in area B, water content becomes an important factor limiting distribution. 3. Other factors such as disturbance by tidal currents and the presence of black sulphide mud are of local importance. 4. Salinity is not a factor governing distribution in this area. 5. In Benfleet Creek the habitat of these two species may be described as follows: — (i) Corophium arenarium inhabits areas where (a) the silt and clay content of the substratum is between 14 % and 27 % (clay content 12 % and 17 % and silt content between 2% and 10%) and the fine sand content is between 76% and 88%; (b) the water content is between 24% and 285%, not usually above 30% and probably not lower than 20%. (c) This species is found on sandy, mud banks, usually confined to the flat tops. Where it is found on the slopes of the banks it avoids the steeper gradients of the lower slopes. (d) Black sulphide mud must be below 5 ems. depth, which is the average length of the burrows. (ii) C orophium v olutat or inhabits areas where (a) the silt and clay content is about 37 % to 38% (clay content between 20% and 25% and silt content between 13% and 16%) and the fine sand content is about 61% to 65 % ; (b) the water content of the mud is about 38% to 39%, probably not much over 40% but as low as 28% for short periods of time. (c) Like C. arenarium it is usually found on the flat tops of the banks and is sensitive to tidal currents. (d) It is never found burrowing in black sulphide mud nor in association with Paludestrina stagnalis. 308 THE ESSEX NATURALIST These descriptions of the habitat refer only to the area studied, i.e. Benfleet Creek. If these observations and deductions are to be confirmed and checked a great deal more work on similar lines in other suitable localities is needed.

REFERENCES ALLEN, E. J. & TODD, R. A. (1900). The fauna of the Salcombe Estuary. J. mar. biol. Ass. U.K., 6: 151-217. BASSINDALE, It. (1938). The Intertidal Fauna of the Mersey Estuary. J. mar. biol. Ass. U.K., 23: 83-98. BASSINDALE, R. (1943). Studies in the Biology of the Bristol Channel, XI. J. Ecol., 31, 1. BEANLAND, F. L. (1939). Sand and Mud Communities in the Dovey Estuary. J. mar. biol. Ass. U.K., 22. CRAWFORD, G. I. (1937). A review of the Amphipod Genus Corophium, with notes on the British species. J. mar. biol. Ass. U.K., 21: 589-630. DALES, R. P. (1950). The reproduction and larval development of Nereis diversicolor. J. mar. biol. Ass. U.K., 29: 321-360. Maas, W. G. (1933). Calcium and the resistance of Nereis to brackish water. Nature, London, 132: 748. ELMHIRST, R. (1931). Studies in the Scottish Marine Fauna-The Crustacea of the sandy and muddy areas of the tidal zone. Proc. roy. Soc. Edinb., 51. FRASER, T. J. (1932). Observations on the fauna and the constituents of an estuarine mud in a polluted area. J. mar. biol. Ass. U.K., 18: 69-86. HART, T. J. (1930). Preliminary notes on the Bionomics of the Amphi- pod Corophium volutator, Pallas. J. mar. biol. Ass. U.K., 16: 761-789. HOLME, N. A. (1949). The fauna of the sand and mud banks near the mouth of the Exe Estuary. J. mar. biol. Ass. U.K., 28: 189-237. HOWES, N. H. (1939). The ecology of a saline lagoon in South East Essex. J. Linn. Soc. (Zook), 40. KNUDSEN, M. (1901). Hydro graphical Tables. Reprinted, Copenhagen, 1953. Nicol., E. A. T. (1953). The ecology of a Salt Marsh. J. mar. biol. Ass. U.K., 20: 203. MOORE, H. B. ez SPOONER, G. M. (1940). The ecology of the Tamar Estuary, VI. An account of the macrofauna of the intertidal muds. J. mar. biol. Ass. U.K., 24: 283-330. PEARSE, A. S. (1928). On the ability of certain marine invertebrates to live in dilute sea-water. Biol. Bull., 54: 5. PERCIVAL, E. (1929). Reports on the fauna of the estuaries of the Tamar and River Lyn. J. mar. biol. Ass. U.K., 16: 81-108. REES, C. B. (1940). A preliminary study of the ecology of a mud flat. J. mar. biol. Ass. U.K., 24: 185-199. REID, D. M. (1929). On some factors limiting the habitat of Areni cola marina. J. mar. biol. Ass. U.K., 16: 109-116. FURTHER SUBSIDENCES NEAR STIFFORD, ESSEX 309

STOPFORD, S. C. (1951). An ecological survey of the Cheshire Foreshore of the Dee Estuary. J. Anim. Ecol., 20: 103-122. SOUTHWARD, A. J. (1953). The fauna of some sandy and muddy shores in the south of the Isle of Man. Proc. Trans. Liv. Biol. Soc., 59. TAIT, J. (1917). Immersion Experiments on Mgia oceanica. Proc. roy. Soc. Edinb., 37: 50. WATKINS, E. E. (1941). The yearly life cycle of the Amphipod Corophium volutator. J. Anim. Ecol., 10: 77-93.

ACKNOWLEDGEMENTS Thanks are extended to Mr. P. Harris for helping with the soil analysis techniques; Mr. G. I. Crawford for confirming my identifications of the two species of Carophium; the laboratory technician of the University College of North Staffs. for determining the salinity; Mr. A. Meadows for the loan of a chemical balance; and Dr. R. G. Evans, Ph.D., F.Z.S., Senior Lecturer in Biology at the University College of North Staffs, for supervising the work.